What are the causes of Horner's syndrome?

Causes of Horner's Syndrome

Horner's syndrome is most commonly caused by carotid artery dissection, accounting for approximately 15% of cases, followed by other vascular pathologies, central nervous system lesions, and iatrogenic causes. 1

Anatomical Understanding

Horner's syndrome results from disruption of the oculosympathetic pathway, which consists of a three-neuron pathway:

1. First-order (central) neuron: Originates in the hypothalamus and extends down the spinal cord
2. Second-order (preganglionic) neuron: Arises from T1-T3 spinal cord segments and travels through the thorax and cervical region to synapse at the superior cervical ganglion
3. Third-order (postganglionic) neuron: Travels from the ganglion to the orbit

Major Causes by Anatomical Location

Central/First-Order Neuron Lesions

• Brainstem stroke or ischemia
• Demyelinating diseases (multiple sclerosis)
• Tumors of the brainstem or cervical spinal cord
• Syringomyelia or syringobulbia

Preganglionic/Second-Order Neuron Lesions

• Pancoast tumor (apical lung cancer)
• Trauma to neck or upper thorax
• Thoracic aortic aneurysm
• Cervical lymphadenopathy
• Thyroid malignancy

Postganglionic/Third-Order Neuron Lesions

• Carotid artery dissection (most common vascular cause) 1
• Carotid artery occlusion or aneurysm 2
• Cavernous sinus thrombosis or tumors
• Middle ear infections
• Cluster headache
• Fibromuscular dysplasia 1

Iatrogenic Causes

• Cervical epidural steroid injections 3
• Scalene blocks and other regional anesthesia 4
• Surgical procedures of the head, neck, and upper thorax

Clinical Presentation and Associated Findings

The classic triad of Horner's syndrome includes:

• Ptosis (drooping of the upper eyelid)
• Miosis (pupillary constriction)
• Anhidrosis (decreased sweating on the affected side of the face)

Important associated symptoms that may indicate specific causes:

• Neck or head pain with carotid artery dissection 5
• Neurological deficits suggesting brainstem involvement 6
• Arm pain or weakness suggesting Pancoast tumor 1
• Vertigo, dizziness, and nausea suggesting vestibular involvement 6

Red Flags Requiring Urgent Evaluation

• New-onset Horner's syndrome with head or neck pain (suspect carotid dissection) 1
• Associated neurological deficits (weakness, sensory loss)
• History of trauma
• Signs of vascular compromise
• Horner's syndrome with internuclear ophthalmoplegia (INO) or other brainstem signs 6

Diagnostic Approach

1. Clinical examination: Confirm the classic triad and look for associated findings

2. Pharmacological testing:

   • Cocaine test (gold standard) to confirm diagnosis
   • 1% phenylephrine to identify postganglionic lesions

3. Imaging based on suspected location:

   • MRA or CTA of the neck for suspected carotid artery dissection
   • Brain MRI for suspected central lesions
   • Chest imaging for suspected Pancoast tumor
   • MRI of the brainstem and cerebellum for suspected skew deviation 6

Treatment Considerations

Treatment is directed at the underlying cause:

• Anticoagulation or antiplatelet therapy for carotid artery dissection 1
• Surgical resection, radiation, or chemotherapy for tumors
• Conservative management for idiopathic cases

Clinical Pearls

• Carotid dissection with Horner's syndrome may lead to cerebral or retinal ischemia in 50-95% of cases if not promptly identified and treated 1
• Idiopathic Horner's syndrome is common but should be a diagnosis of exclusion
• New-onset Horner's syndrome should always prompt investigation for malignancy in adults with risk factors 1
• Horner's syndrome associated with acute pain requires urgent vascular imaging to rule out carotid dissection 5

Understanding the anatomical pathway of the sympathetic innervation to the eye is crucial for localizing the lesion and determining the underlying cause of Horner's syndrome.